A laboratory fume hood is a ventilated enclosure where harmful materials can be handled safely. The hood captures contaminants and prevents them from escaping into the laboratory by using an exhaust blower to draw air and contaminants in and around the hood's work area away from the operator so that inhalation of and contact with the contaminants are minimized. Access to the interior of the hood is through an opening which is closed with a sash which typically slides up and down to vary the opening into the hood.
The velocity of the air flow through the hood opening is called the face velocity. The more hazardous the material being handled, the higher the recommended face velocity, and guidelines have been established relating face velocity to toxicity. Typical minimum face velocities for laboratory fume hoods are 75 to 150 feet per minute (fpm), depending upon the application.
When an operator is working in the hood, the sash is opened to allow free access to the materials inside. The sash may be opened partially or fully, depending on the operations to be performed in the hood. While fume hood and sash sizes vary, the opening provided by a fully opened sash is on the order of ten square feet. Thus the maximum air flow which the blower must provide is typically on the order of 750 to 1500 cubic feet per minute (cfm).
The sash is closed when the hood is not being used by an operator. It is common to store hazardous materials inside the hood when the hood is not in use, and a positive airflow must therefore be maintained to exhaust contaminants from such materials even when the hood is not in use and the sash is closed.
It is important that the face velocity be kept as constant as possible. The minimum acceptable face velocity is determined by the level of hazard of the materials being handled, as discussed above. Too high a face velocity may cause turbulence, however, which can result in contaminants escaping from the hood. Additionally, high face velocities can be annoying to the operator and can damage fragile apparatus in the hood. As the hazard level of the materials being handled and the resulting minimum face velocity increases, maintaining a safe face velocity becomes more difficult.
Another important consideration in the design of a fume hood system is the cost of running the system. There are three major areas of costs: the capital expenditure of installing the hood, the cost of power to operate the hood exhaust blower, and the cost of heating, cooling, and delivering the "make-up air," which replaces the air exhausted from a room by the fume hood. For a hood operating continuously with an opening of 10 square feet and a face velocity of 100 fpm, the cost of heating and cooling the make-up air, for example, could run as high as fifteen hundred dollars per year in the northeastern United States. Where chemical work is done, large numbers of fume hoods may be required. For example, the Massachusetts Institute of Technology has approximately 650 fume hoods, most of which are in operation 24 hours per day.
Reliability is another important factor in the design of a fume hood system. It is important that the face velocity of a fume hood not be allowed to go below a certain level. The amount of air being exhausted from a hood may be decreased by many common occurrences; duct blockage, fan belt slippage or breakage, deterioration of the blower blades, especially where corrosive materials are being handled, motor overload, and other factors. A reduction in air flow reduces the face velocity, and it is important to take immediate steps when a low flow condition occurs to prevent escape of contaminants from the hood.
A conventional fume hood consists of an enclosure which forms five sides of the hood and a hood sash which slides up and down to provide a variablesized opening on the sixth side. In this type of hood, the amount of air exhausted by the hood blower is essentially fixed, and the face velocity increases as the area of the sash opening decreases. As a result, the sash must be left open an appreciable amount even when the hood is not being used by an operator to allow air to enter the hood opening at a reasonable velocity.
To maintain a more constant face velocity as the hood sash is moved up and down, so-called "by-pass" hoods have been developed. A by-pass hood has a by-pass opening through which air can enter the fume hood. The by-pass opening is blocked by the sash when it is in the fully opened position. As the sash is lowered, the by-pass opening is gradually uncovered so that air can "by-pass" the hood opening and enter the hood directly, thus preventing the air velocity through the hood opening from becoming too high as the sash is closed.
In known types of fume hoods having a fixed fan speed, the air flow in the hood system may be monitored, for example by means of a flow sensor in the exhaust duct, to determine if the air flow and hence face velocity is below a selected value. It has proven difficult to provide sensors which reliably monitor the performance of a fume hood exhaust system. Air flow sensors are costly and non-linear. They are also subject to contamination by the materials in the exhaust air. Pressure sensors are difficult to use because of the very low pressure drops which can exist in the exhaust ducting if the air flow is varied.
Both conventional and by-pass hoods exhaust a fixed amount of air from the room regardless of sash position. As discussed above, the resulting loss of air from the room can waste a lot of energy. To minimize this loss, so-called "add-air" hoods have been developed. An add-air hood includes an additional blower and duct system which supplies air directly to the front of the hood from outside to provide a portion of the make-up air.
Add-air hoods have not proven to be as successful as might be expected at reducing operating costs. The initial installation expense for such hoods is much higher. Additionally, since the make-up air usually requires conditioning to provide reasonable operator comfort, the heating and cooling costs that are saved are often very modest. Furthermore, many conventional and by-pass hoods installations exist which were installed before the recent dramatic increase in energy prices, and adding the extra ducting and associated equipment required by add-air hoods to existing installations can be extremely expensive.